I'm contemplating a world in which there is some sort of cyclical darkness that covers a continent or the whole planet, and humanity has to adjust to deal with the creatures that surface when the darkness comes.
Something more than just the day/night we have on earth. Perhaps the lunar cycle is closer. For example, every 28 days the planet (or a continent) is covered in darkness for 3 days.
I'm not referring to one side of the planet permanently being dark, such as a tidal locked planet.
Other than astronomical causes, would there be anything else that could cause this? Fog? Haze?